The work proposed in this application is on visual systems where there is particular promise for enriching our broad understanding of the basic biochemistry, physiology, and cell biology of early events in photoreception and vision. Non-human species can sometimes be used as models, and sometimes they can help to show us what is ubiquitous and essential. A major step in the biochemical cycle of all visual systems is the isomerization of retinoids from the all-trans to the 11-cis configuration. There is considerable diversity in the manner in which this is accomplished in the major groups of animals, but some crustaceans appear, surprisingly, to have solved this problem in a way that is very similar to that of humans and other vertebrates. The immediate aims of this component of the work are i) development of an in vitro system for the formation of 11-cis retinol in extracts of eyes of the lobster (Homarus); ii) establish for lobster the relationships between retinyl ester synthetase activity and the trans-to-cis isomerization process for retinoids; iii) examine the turnover of retinoid stores during light and dark adaptation in living lobsters; and iv) explore the possible occurrence of the lobsters isomerization system in other arthropods. Vitamin A is derived from carotenes with at least one beta-ionone ring. For vertebrates, the hydroxylated carotenoids called xanthophylls lack pro-vitamin A activity. Certain insects, however, utilize 3- hydroxyretinal as chromophore of their visual pigments, and another set of experiments is designed to test the limits of Diptera to utilize dietary carotenoids of different structures as precursors of 3-hydroxyretinal and 3-hydroxyretinol, analyzing extracts of the eye by HPLC. In the last few years, many lower vertebrates have been shown to have cones maximally sensitive in the near UV. A third focus of this study is to characterize these cones with regard to absolute and spectral sensitivity by recording photocurrents from single cells. The possible participation of these cones in a tetrachromatic color vision system will be tested in related behavioral experiments on birds.